Condensed Consolidated Balance Sheets Parenthetical - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Fixed maturities, amortized cost	$ 21,924.5	$ 22,364.8
Equity securities, cost	69.2	67.1
Mortgage loans on real estate valuation allowance	0.9	1.1
Securities pledged, amortized costs	$ 944.4	$ 692.8
Common stock, par value (USD per share)	$ 50	$ 50
Common stock, shares authorized	100,000	100,000
Common stock, shares issued	55,000	55,000
Common stock, shares outstanding	55,000	55,000